THE FLIES 45 



advanced and within a few hours enter upon the pupal stage which re- 

 quires from six to eight weeks. Occurring only in Africa, they are most 

 abundant in heavily wooded districts penetrated by water courses. 

 Both sexes are blood-sucking, and it is in such locations that they are 

 most likely to find the wild animals upon which they feed. 



Relationship to Trypanosomiasis. — As transmitters of trypanoso- 

 miasis to man and domestic animals, tsetse flies may be regarded as the 

 world's most dangerous insects. The first observation of trypanosomes 

 in the blood of mammals was made by Lewis, who in 1877 described a 

 trypanosome {Trypanosoma lewisi) of the blood of a rat. Three years 

 later another trypanosome {T. evansi) was studied as the cause of surra 

 in horses. WhenBruce in 1894 demonstrated the relationship between 

 tsetse fly disease of horses in Africa, the cause of which was unknown, 

 and nagana, trypanosomes received much more attention as to their 

 pathogenic importance. The further investigations of Bruce as to the 

 part played by the tsetse fly in the transmission of this disease are best 

 given in his own accoimt, from which the following is an excerpt: 



"When it was once established that the two diseases were the same, 

 experiments were made to find out how the animals became infected, 

 whether the fly was the carrier or the mere concomitant of the low-lying, 

 imhealthy district, and, if a carrier, if it was the only carrier of the disease 

 from sick to healthy animals. Horses taken down into the fly coimtry, 

 and not allowed to feed or drink there, took the disease. Bimdles of 

 grass and supplies of water, brought from the most deadly parts of the 

 fly coimtry to the top of Ubombo and there used for fodder for healthy 

 horses failed to convey the disease. Tsetse flies caught in the low country 

 and kept in cages on top of the moimtain, when fed on affected animals, 

 were capable of giving rise to the disease in healthy animals up to forty- 

 eight hours after feeding. Tsetse flies brought up from the low country 

 and placed straight way upon healthy animals were also found to give 

 rise to the disease. The flies were never found to retain the power of 

 infection for more than forty-eight hours after they had fed upon a sick 

 animal, so that if wild tsetse flies were brought up from the low country, 

 kept without food for three days, and then fed on a healthy dog, they 

 never gave rise to the disease. In this way it was proved that the tsetse 

 fly, and it alone, was the carrier of nagana. Then the question arose as 

 to where the tsetse flies obtained the trypanosomes. The flies Hved 

 among the wild animals, such as buffaloes, koodoos, and other species of 

 antelopes, and naturally fed on them. It seemed that, in all probability, 

 the reservoir of the disease was to be foimd in the wild animals. There- 

 fore, all the different species of wild animals obtainable were examined 

 both by the injection of their blood into healthy susceptible animals, 

 and also by direct microscopic examination of the blood itself. In this 

 way it was discovered that many of the wild animals harbored this 



